1. Technical Field
This invention generally relates to computer systems, and more specifically relates to compilers that generate executable code for computer systems.
2. Background Art
Since the dawn of the computer age, computer systems have evolved into extremely sophisticated devices, and computer systems may be found in many different settings. Dramatic advances in both hardware and software (e.g., computer programs) have drastically improved the performance of computer systems. Modern software has become very complex when compared to early computer programs. Many modem computer programs have tens or hundreds of thousands of instructions. The execution time (and hence, performance) of a computer program is very closely related to the number of instructions that are executed as the computer program runs. Thus, as the size and complexity of computer programs increase, the execution time of the computer program increases as well.
Unlike early computer programs, modern computer programs are typically written in a high-level language that is easy to understand by a human programmer. Special software tools known as compilers take the human-readable form of a computer program, known as “source code”, and convert it into “machine code” or “object code” instructions that may be executed by a computer system. Because a compiler generates the stream of machine code instructions that are eventually executed on a computer system, the manner in which the compiler converts the source code to object code affects the execution time of the computer program.
The execution time of a computer program, especially complex computer programs, is a function of the arrangement and type of instructions within the computer program. Branch instructions are one type of instruction that may affect the execution time of a computer program. A branch instruction may cause the processor to go (i.e., branch) to a different part of the computer program to execute instructions. If a computer program contains many branch instructions, the time spent branching from one portion of the program to the next may introduce delays. Judicious selection of the order of portions of the computer program may improve the program's execution time by placing portions that are executed sequentially in sequential order.
In order to optimize the performance of modern computer programs, profilers have been developed to predict and/or measure the run-time performance of a computer program. Profilers typically generate profile data that estimates how often different portions of the computer program are executed. Using profile data, an optimizer (such as an optimizing compiler) may make decisions regarding the preferred order of different portions of the computer program in order to optimize the execution speed of the computer program.
Known prior art systems generate profile data that is used by a compiler to determine the order of portions of a computer program. However, the known methods for using profile data in reordering portions of a computer program do not provide an optimal solution. For example, in the prior art, virtual method calls in an object oriented computer program cannot be optimized because the compiler does not know at compile-time which method at run-time will be the actual method being invoked. As a result, the prior art may still yield inefficiencies in the structure of the code that result in a slower execution time for the computer program. Without improved apparatus and methods for devirtualizing virtual method calls in an object oriented computer program, these method calls will remain unaffected by any optimization performed by the compiler, resulting in a computer program that is not as fully optimized as it might be.